Pressureless sintering of whisker-toughened ceramic composites

ABSTRACT

A pressureless sintering method is disclosed for use in the production of whisker-toughened ceramic composites wherein the sintered density of composites containing up to about 20 vol. % SiC whiskers is improved by reducing the average aspect ratio of the whiskers to from about 10 to about 20. Sintering aids further improve the density, permitting the production of composites containing 20 vol. % SiC with sintered densities of 94% or better of theoretical density by a pressureless sintering method.

The U.S. government has rights in this invention pursuant to contractNo. DE-AC05-840R21400 awarded by U.S. Department of Energy contract withMartin Marietta Energy Systems, Inc.

This application is a continuation of application Ser. No. 579,506 filedSep. 10, 1990, now abandoned, which is a continuation of parentapplication Ser. No. 340,625 filed Apr. 19, 1989, now abandoned.

The present invention relates generally to methods for the preparationof whisker-toughened ceramic composites and more particularly relates tomethods of pressureless sintering of such composites.

It is known from the Assignee's U.S. Pat. No. 4,652,413 entitled "Methodfor Preparing Configured Silicon Carbide Whisker-Reinforced AluminaCeramic Articles" that whisker-toughened ceramics may be produced by apressureless sintering process to enable the fabrication of articleswith relatively complicated geometries having near final dimensions.This development has significantly expanded the range of configurationof articles incorporating whisker-toughened ceramic composites over theprevious hot pressing techniques which were limited to the fabricationof relatively simple shapes that could be converted to complex shapesonly through extensive post-formation machining processes.

In accordance with the above-referenced patent, the disclosure of whichis incorporated herein by reference, ceramic composites produced bypressureless sintering must contain a SiC whisker concentration of nomore than 10 vol. % in order to achieve desirable sintered densities ofabout 94% of the theoretical density. It was found that compositescontaining greater than 10 vol. % SiC whiskers may not be sinterablewithout pressure assistance. In particular, the tests showed thatcomposites incorporating 20 vol. % SiC whiskers achieved densities nogreater than about 75% of the theoretical density. This limitation onthe permissible concentration of SiC whiskers which may be incorporatedinto ceramic composites produced by a pressureless sintering process hasinhibited full realization of the advantages available through the useof SiC whiskers in items produced from such methods.

It is, therefore, an object of the present invention to provide a methodfor making whisker-toughened ceramic composites by pressurelesssintering.

An additional object of the invention is to provide a pressurelesssintering method for the production of whisker-toughened ceramiccomposites containing up to about 20 vol. % SiC whiskers wherein thecomposites exhibit improved sintered densities.

Yet another object of the invention is the provision of a pressurelesssintering method for use in the production of whisker-toughened ceramiccomposites incorporating up to about 20 vol. % SiC whiskers wherein thesintered densities are at least about 94% of theoretical.

Still another object of the invention is to provide a pressurelesssintering method for whisker-toughened ceramics incorporating aluminaand mullite as the ceramic matrix material.

A further object of the invention is the provision of a pressurelesssintering method for whisker-toughened ceramics which employs new andbeneficial liquid-phase sintering aids.

The foregoing and other objects and advantages are achieved inaccordance with the present invention which provides for the productionof whisker-toughened ceramic composites containing up to about 20 vol. %SiC whiskers by a pressureless sintering method wherein the compositesexhibit improved sintered densities over previous composites produced bypressureless sintering methods incorporating correspondingconcentrations of whiskers. The method is applicable to the productionof ceramic composites incorporating a ceramic matrix material selectedfrom the class consisting of alumina and mullite, SiC whiskers, and asintering aid selected from the class consisting of Y₂ O₃, MgO, ZrO₂,Si₃ N₄ and mixtures thereof. In general, the method comprises forming ahomogeneous mixture including up to about 20 vol. % of SiC whiskers withthe whiskers having an average aspect ratio (length/diameter) of fromabout 10 to about 20, a sintering aid selected from the class consistingof Y₂ O₃, MgO, ZrO₂, Si₃ N₄ and mixtures thereof in a sufficientconcentration to provide liquid phase sintering, and a ceramic matrixmaterial selected from the group consisting of alumina and mullite. Themixture is shaped into a preselected configuration at a pressed densityin the neighborhood of about 50 % of the theoretical density and thenheated at essentially atmospheric pressure in an inert atmosphere at atemperature sufficient to effect liquid-phase sintering of the ceramicmatrix to provide a self-supporting composite formed of the ceramicmatrix material toughened by a dispersed whisker phase therein with animproved sintered density over previous composites produced bypressureless sintering containing corresponding concentrations of SiCwhiskers.

Typically, the whiskers in the as-received condition have an averageaspect ratio of greater than about 50. It is found that the substantialimprovements in sintered densities disclosed herein for 20 vol. % SiCwhisker composites are achieved by reducing the aspect ratio to fromabout 10 to about 20, permitting the production of sintered compositeswithout pressure assistance.

Through an appropriate combination of constituents and concentrations,sintered densities of at least about 94% of the theoretical density areachieved in composites containing up to 20 vol. % SiC whiskers producedby the pressureless sintering method of the invention. For example, in apreferred embodiment, alumina is selected as the matrix material and thewhiskers which are in the typical as-received condition having anaverage aspect ratio over about 50 are processed to reduce the averageaspect ratio to from about 10 to about 20 and are incorporated in aconcentration of from about 10 to about 20 vol. %. The sintering aid isa mixture of yttria and MgO with the yttria being provided in aconcentration of from about 10 to about 2 vol. % and the MgO in aconcentration of from about 0.1 to about 1 wt. %. And the sinteringtemperature is maintained in the range of from about 1750° to about1800° C. The resulting composites exhibit a sintered density approachingor exceeding 94% of the theoretical density.

The invention will now be described in further detail with reference tothe following detailed description in conjunction with the accompanyingdrawings in which:

FIG. 1 is a graphical illustration showing SiC whisker lengthdistribution for various milling durations;

FIG. 2 is a graphical illustration showing sintered densities as apercentage of theoretical density for ceramic composites incorporating20 vol. % SiC whiskers with different aspect ratios;

FIG. 3 is a graphical illustration plotting yttria content versussintered density as a percentage of theoretical density for 20 vol. %SiC whisker ceramic composites at various sintering temperatures;

FIG. 4 is a graphical illustration plotting percent liquid phase versussintered density as a percentage of theoretical density for 20 vol. %SiC whisker composites at various sintering temperatures;

FIG. 5 is a graphical illustration showing pre-sintering andpost-sintering densities as a percentage of theoretical density formullite-10 vol. % SiC whisker composites with and without sinteringaids; and

FIG. 6 is a graphical illustration plotting sintering temperature versussintered density as a percentage of theoretical density for mullite-SiCwhisker composites containing various concentrations of SiC whiskers.

The method of the present invention is applicable to the production ofwhisker-toughened ceramic composites incorporating a ceramic matrixmaterial such as alumina and mullite. As described in Assignee's U.S.Pat. No. 4,652,413, the alumina powder is preferably in a size range offrom about 0.1 to about 1 micrometer. The particulate mullite when usedas the ceramic matrix ingredient preferably is in a size range of fromabout 0.1 to about 3 micrometers. Mullite is a desirable constituent incertain applications since mullite-based composites have been found toexhibit lower thermal conductivities, lower coefficients of thermalexpansion, lower densities, and improved thermal shock and creepresistance relative to the alumina-based forms.

The SiC whiskers are described in detail in U.S. Pat. No. 4,543,345entitled "Silicon Carbide Whisker-Reinforced Composites and Method forMaking Same", which is incorporated herein by reference. The whiskersdisclosed in the aforementioned patent have a monocrystalline structureand, in the as-received form, have a diameter of about 0.6 micrometersand a length in the range of from about 10 to about 80 micrometers. Thediameter of other commercially available SiC whiskers may vary fromabout 0.1 to about 2 micrometers and the length of the whiskers may varyfrom 10 to 100 micrometers. However, the SiC whiskers in all of thevarious commercially available forms generally have an average aspectratio in excess of about 50 in the as-received form.

It is a feature of the invention that the average aspect ratio of thewhiskers incorporated into the composites is in the range of from about10 to about 20 to enable heretofore unachievable sintered densities incomposites having up to 20 vol. % of SiC whiskers. As described morefully hereinafter, this average aspect ratio of from about 10 to about20 is accomplished by milling the whiskers preferably with the otheringredients to thereby reduce the as-received average aspect ratio ofabout 50 to within the necessary range.

A particularly preferred liquid phase sintering aid is a mixture ofyttria and MgO in which the yttria is incorporated within a permissiblerange of from about 2 to about 30% and a preferred range of from about10 to about 30 wt. %, with a yttria concentration range of from about 10to about 20 wt. % being the most preferred. The MgO is incorporated in apreferred range of from about 0.1 to about 2 wt. % with a concentrationof about 0.5 wt. % being particularly preferred. The Si₃ N₄ and ZrO₂constituents when used as sintering aids are preferably incorporated inthe range of from about 1 to about 20 wt. % and from about 1 to about 20vol. %, respectively. The Si₃ N₄ is most effective as a sintering aidwhen used with yttria in a concentration of from about 0.5 to about 5wt. % Si₃ N₄ and from about 2 to about 5 wt. % yttria. The ZrO₂, on theother hand, is most effective when used alone as a sintering aid in aconcentration of from about 5 to about 20 vol. %.

In the course of preparing the composites for sintering, theconstituents are blended in a suitable manner such as in an ultrasonichigh-shear mixer of a suitable, commercially available type. Blendingtimes may vary, although a duration of about 1 to 5 minutes usuallyprovides a sufficiently homogeneous mixture. Upon completion of theblending the resulting mixture is preferably subjected to a wet millingin a ball mill operation to break up any particulate agglomerates andimprove the packing efficiency of the mixture.

The present invention departs from known prior processes in that themilling is carried out for up to 32 hours and preferably in the range offrom about 16 to 32 hours. That is, in the prior processes millingdurations of from about 0.5 to about 8 hours maximum have been practicedin order to lower the size distribution of the SiC whiskers and improvetheir packing efficiency for providing a final product with the desiredhigh density and fracture toughness. It was believed that compositeswith whisker concentrations exceeding about 10 vol. % could not besintered to produce acceptable densities without pressure assistance,regardless of the extent to which the ingredients were milled. However,it has been discovered in accordance with the present invention thatmilling the SiC whiskers sufficient to achieve an average aspect ratioof from about 10 to about 20 enables the production of compositesexhibiting significantly improved sintered densities to permit thefabrication of relatively complex shapes containing an increasedconcentration of SiC whiskers and thus improved fracture toughness.Preferably, then, the SiC whiskers are incorporated in a concentrationof about 20 vol. and the whiskers are milled for at least about 16 hoursto reduce the average aspect ratio to from about 10 to about 20.

Depending on the condition of the matrix material and sintering aid itmay be desirable to mill these constituents separately prior to theadmixture of the SiC whiskers, such as where the constituents appear tocontain substantial particulate agglomerates, clumps and the like. Theas-received SiC whiskers are then added and the mixture milled for anappropriate length of time to achieve an aspect ratio of the whiskers inthe range of from about 10 to 20.

Once the mixture is blended and homogenized as a result of the millingprocess and the necessary aspect ratio of the SiC whiskers is achieved,the dried mixture is placed in a suitable mold or other shape formingmechanism and subjected to a pressure of from about 10,000 to about60,000 psi to achieve a "green" density in the neighborhood of 50% ofthe theoretical density. The pressed powder compact is then placed on asuitable support compatible with the expected sintering temperatures,and at this time may be covered with an excess SiC whiskers or SiCpowder to promote sintering of the composite. The loaded support is thenplaced in a suitable furnace and heated in an atmosphere of inert gassuch as argon or nitrogen to a temperature sufficient to effectpressureless liquid-phase sintering of the particulates defining thematrix containing the dispersed SiC whiskers. The sintering temperatureis preferably in the range of from about 1700° to about 1850° C., and ismost preferably about 1800° C. for assuring sintering uniformity. It ispreferred that the compact be heated at a relatively rapid rate in theorder of about 90 minutes from room temperature to about 1000° C. andthen at a slightly slower rate in the order of about 60 minutes fromabout 1000° C. to 1800° C. The compact is then exposed to the finaltemperature for a hold time of from about 1 to about 30 minutes toeffect complete sintering.

The pressureless sintering process is effective to provide aself-supporting structure of the sintered ceramic in a relativelycomplicated geometry and containing an increased SiC whiskerconcentration of up to 20 vol. % with the composite exhibitingsignificantly improved sintered densities over prior composites producedby pressureless sintering and containing corresponding concentrations ofSiC whiskers. In particular, it is found that by maintaining the aspectratio of the SiC whiskers in the range of about 10 to about 20 asaforesaid that pressureless sintered densities of at least about 94% ofthe theoretical density are achieved for 20 vol. % SiC whiskercomposites when the sintering aid is a mixture of yttria and MgO in aconcentration of from about 10 to about 20 wt. % and about 0.5 wt. %,respectively, effective to achieve an appropriate amount of a liquidphase to promote the sintering process. Previously reported work showeda maximum obtainable sintered density equal to no greater than about 75%of the theoretical density for pressureless sintered compositesincorporating 20 vol. % SiC whiskers.

The sintered composites produced by the invention exhibiting improveddensities including the particularly desirable density level of at leastabout 94% of theoretical may be suitable for some uses without the needof further densification treatments. Such uses include cutting tools,wear parts and heat engine components. However, in some instances it maybe necessary to further densify the composite to achieve a density ofabout 98% or greater of theoretical density. In such cases, thecomposite may be subjected to hot isostatic pressing in an inertatmosphere at a temperature in the range of from about 1600° to about1700° C. and a pressure loading in the range of from about 10,000 toabout 30,000 psi. It is noted that the hot isostatic pressing operationcan be achieved without encapsulating the composite since the compositepossess sufficient structural integrity as a result of the previouspressureless sintering step to withstand the hot isostatic pressingoperation. Normally, the hot isostatic pressing step is carried out overa duration of from about 0.1 to about 1 hour to produce a final densityof at least about 98% of the theoretical density.

The following examples further illustrate the invention and are not tobe considered as placing any limitations thereon.

EXAMPLE I

Alumina based composites containing 20 vol. % SiC whiskers, 0.5 wt. %MgO and 2 wt. % yttria are prepared by blending the alumina, MgO, andyttria for 3 minutes and then ball milling the mixture for about 16hours. The SiC whiskers which have an as-received average aspect ratioof about 50 are then added and separate portions are milled for 4, 8, 16and 32 hours. The resulting whisker length distribution obtained by themilling procedure is illustrated in FIG. 1 which shows considerablereduction in the whisker length during milling. Specifically, for amilling duration of 4 hours the average aspect ratio of the whiskers isabout 25. After 8 hours, the average aspect ratio is reduced to justabove about 20 and after 16 hours the average aspect ratio drops belowabout 20 to an average of about 18. Milling for a total of 32 hoursreduces the average aspect ratio to about 17.

After the milling procedure is completed, the dried powder mixtures arepressed at about 200 Mpa for a duration of 1 minute to producecomposites having a green density of from about 55 to about 65% oftheoretical density. Separate samples from each portion are then heatedat atmospheric pressure in a graphite-element furnace under argon gas at1700°, 1800° and 1850° C. FIG. 2 shows the dramatic improvement insintered density as a percentage of the theoretical density as theaspect ratio is lowered to from about 10 to about 20 for the aluminacomposites including 20 vol. % SiC whiskers.

EXAMPLE II

Alumina-SiC whisker composites are prepared as described in Example Iexcept that the yttria content is varied between 2 and 30 wt. % with theMgO concentration being held constant at 0.5 wt. %. A portion of thesamples containing 2 wt. % yttria are milled 16 hours following additionof the SiC whiskers. All other samples are milled 32 hours after the SiCwhiskers are added. The samples are heated at temperatures of from 1700°to 1850° C. to produce composites having sintered densities as apercentage of theoretical density (T.D.) as shown in Table I below. Itis noted that the 2 wt. % yttria samples milled 16 hours were heatedonly at 1700° and 1800° C. and that the remaining 2 wt. % yttria samplesthat were milled 32 hours were heated at 1750° C.

                  TABLE I                                                         ______________________________________                                        Wt. %   Sintering              Calculated                                     Y.sub.2 O.sub.3                                                                       Temp.         % T.D.   % Liquid                                       ______________________________________                                         2      1700          74.5      4                                             10      1700          90.0     21                                             20      1700          94.3     40                                             30      1700          96.0     58                                              2      1750          84.3      5                                              5      1750          92.5                                                    10      1750          94.1     23                                             20      1750          93.3     45                                             30      1750          92.3     65                                              2      1800          82.7      6                                             10      1800          93.5     28                                             20      1800          92.0     54                                             30      1800          90.0     78                                              2      1850          89.7                                                    ______________________________________                                    

Data from Table I is reproduced onto the graph of FIG. 3 for samplescontaining from about 2 to about 30% yttria and heated at temperaturesof 1700°, 1750° and 1800° C. FIG. 3 shows that the sintered densitiesgenerally increase with both temperature and yttria content up to about10 vol. yttria for sintering temperatures of 1750° and 1800° C. and thatsintered densities at a sintering temperature of 1700° C. increasesteadily with increasing yttria content. At temperatures equal to orgreater than 1750° C. and for yttria contents greater than about 10 wt.% the densities are observed to decrease slightly as the yttria contentis increased. This effect is believed to be due to excessive liquidphase formation at higher yttria contents and higher temperaturesleading to increased decomposition and volatilization of gaseousspecies. This is illustrated in FIG. 4 which compares the % liquid phaseversus sintered density at the temperatures and yttria contents shown inTable I and depicted in FIG. 3. From FIG. 4 it is seen that densities donot generally improve with liquid phase contents greater than about 30to 40%. From FIG. 3 it is seen that sintered densities approaching orexceeding about 94% of the theoretical density are achieved with yttriaconcentrations in the range of from about 10 to about 20 wt.

EXAMPLE III

A sample is prepared containing alumina, 20 vol. % SiC whiskers and 12vol. % ZrO₂. The alumina and ZrO₂ are ball milled for 16 hours prior toaddition of the SiC whiskers after which the mixture is ball milled foran additional 4 hours. The milled mixture is dried and then pressed at380 Mpa to a green density of about 55% of theoretical density, and isthen heated at atmospheric pressure under nitrogen at 1700° C. The firedcomposite has a density of about 90% of the theoretical density.

EXAMPLE IV

Samples are prepared containing mullite, SiC whiskers and in some casesyttria and MgO as sintering aids. In a first group of samples thecomposites incorporate 10 vol. % SiC whiskers. Half of the samplescontain 2 wt. % yttria and 0.5 wt. % MgO as sintering aids. The othersamples contain no sintering aid. In preparing the samples, the mulliteand, when present, the sintering aids, are all ball milled for 24 hoursprior to admixture of the SiC whiskers after which the mixture is milledan additional 16 hours. The resulting dried mixtures are pressed at 200Mpa for 1 minute and the resulting density is measured as the greendensity. The samples are then heated at atmospheric pressure an argonatmosphere at 1600° C. The results are depicted in FIG. 5 which showsthat the sintered density of the mulltie-10 vol. % SiC whiskercomposites exceed 95% of theoretical upon addition of the sinteringaids.

In the second group of mullite samples, a sintering aid is incorporatedconsisting of 2 wt. % yttria and 0.5 wt. % MgO. The SiC whisker contentis varied to produce mullite composites having 10, 15 and 20 vol. % SiCwhiskers. The mullite and sintering aid is pre-milled for 24 hours priorto admixture of the SiC whiskers after which the mixture is ball milledan additional 16 hours. The dried mixtures are pressed at 200 Mpa to agreen density of from about 55 to about 65% of the theoretical densityand then heated under argon gas at atmospheric pressure at temperaturesof 1500°, 1600°, 1700°, 1750° and 1800° C. The results are shown in FIG.6.

It is observed from FIG. 6 that the sintered density for mullitecomposites decreases with increasing whisker content and that for allwhisker concentrations the sintered density generally increases with thesintering temperature. The best sintered densities are achieved in themullite composites containing 10 vol. % SiC whiskers which are sinteredat temperatures of from about 1600° to about 1700° C.

EXAMPLE V

Two alumina composites are prepared, the first consisting of aluminatogether with 10 vol. % SiC whiskers and 2 wt. % yttria, and the secondbeing identical to the first except for the addition of 2 wt. % Si₃ N₄.For each sample, all of the constituents except the whiskers are ballmilled for 16 hours before admixture of the SiC whiskers, after whichthe resulting mixture is milled an additional 8 hours. Compacts of eachsample are pressed at 380 Mpa for an appropriate time to achieve a greendensity of 58% of the theoretical density, followed by heating at 1700°C. in a nitrogen atmosphere at atmospheric pressure. The fired densitiesare 88.2% of theoretical density and 93.1% of the theoretical density,respectively.

What is claimed is:
 1. The method of producing whisker-toughened ceramiccomposites from alumina, SiC whiskers having an aspect ratio of at leastabout 50, and a sintering aid including yttria and MgO, which comprisesforming a homogeneous mixture including from about 10 to about 20 wt. %of the SiC whiskers, a yttria concentration of from about 10 to about 40wt. %, a MgO concentration of from about 0.1 to about 1 wt. %, and abalance of alumina; milling said homogeneous mixture for a period of 16to 32 hours to achieve an average aspect ratio of the whiskers of fromabout 10 to about 20; shaping the mixture into a preselectedconfiguration at a pressed density of about 50% of the theoreticaldensity; and heating the mixture at essentially atmospheric pressure inan inert atmosphere at a temperature in the range of from about 1750° toabout 1800° C. for a sufficient time to effect liquid-phase pressurelesssintering of the ceramic matrix wherein a sintered self-supportingcomposite of the predetermined configuration is produced which istoughened by a dispersed whisker phase and which exhibits an improvedsintered density of at least about 94% of the theoretical density. 2.The method according to claim 1, wherein the average aspect ratio of thewhiskers is reduced and the homogeneous mixture is formed by adding thewhiskers to a mixture of the alumina and the sintering aid, and thenmilling the whiskers together with the alumina and sintering aid underconditions sufficient to achieve an average aspect ratio of the whiskersof from about 10 to about
 20. 3. The method of preparing SiC whiskertoughened ceramic composites comprising: forming a homogeneous mixtureincluding about 10 vol. % to about 20 vol. % SiC whiskers having anaverage as-received aspect ratio of greater than about 50, a sinteringaid selected from the class containing of Y₂ O₃, MgO, ZrO₂, Si₃ N₄, andmixtures thereof in a concentration sufficient to promote liquid phasesintering, and a ceramic matrix material selected from the group aluminaand mullite; milling said mixture for a period of 16 to 32 hours toachieve an average aspect ratio of said SiC whiskers of less than 20;shaping said homogeneous mixture into a preselected configuration at apressed density of about one-half the theoretical density; and heatingthe shaped mixture at essentially atmospheric pressure in an inertatmosphere to a temperature sufficient to effect liquid-phase sinteringto provide a self-supporting composite of the ceramic matrix materialtoughened by a dispersed whisker phase therein with the compositeexhibiting an improved sintered density over previous compositesproduced by pressureless sintering containing correspondingconcentrations of SiC whiskers.
 4. The method of claim 3 wherein thesintering aid is a mixture of Y₂ O₃ and MgO provided in a concentrationof from about 10 to about 40 wt. % Y₂ O₃ and from about 0.1 to about 1wt. % MgO.
 5. The method of claim 3 wherein the sintered density is atleast about 94% of theoretical density.
 6. The method of claim 3 whereinthe sintering temperature is from about 1700° to about 1850° C.
 7. Themethod of claim 3 wherein the sintering temperature is from about 1750°to about 1800° C.